The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for accelerating MRI by making use of spectral sensitivity.
Magnetic resonance imaging (“MRI”) of non-ferrous metallic implants is challenging because of the substantial inhomogeneity induced in the B0 magnetic field of the MRI system. This inhomogeneity leads to severe off-resonance in nearby tissue and distorts conventional spatial-encoding mechanisms. The image artifacts resulting from the off-resonance can significantly degrade the diagnostic quality of an image, making clinical diagnoses in the presence of metal very challenging.
Methods such as view angle tilting (“VAT”), slice-encoding for metal artifact correction (“SEMAC”), and multi-acquisition variable-resonance image combination (“MAVRIC”) have been developed in an attempt to mitigate the off-resonance artifacts surrounding metallic implants. These methods are described, for example, by K. M. Koch, et al., in “Magnetic Resonance Imaging Near Metal Implants,” J Magn Reson Imaging, 2010; 32(4):773-787.
Due to hardware limitations, a single radio frequency (“RF”) pulse is often incapable of exciting the wide range of frequencies near metal. To cover this broad spectrum of frequencies, methods such as MAVRIC utilize multiple acquisitions, each with an RF pulse at different center frequency offsets. Therefore each acquisition produces images with a unique spectral sensitivity pattern. These “spectral” images can then be combined to generate a composite image with signal from all the frequencies. Although MAVRIC is capable of mitigating artifacts caused by large perturbations in the B0 field, it requires long scan times and is, thereby, limited in spatial resolution.
MAVRIC can make use of a number of techniques to reduce the scan time, such as interleaving different RF excitations, partial Fourier, parallel imaging with multiple independent RF coils, and adaptive phase encoding. Adaptive phase encoding employs an effective undersampling by reducing the field-of-view (“FOV”) for the high frequency spectral bins. Significant scan time reduction is possible with this approach, but it requires that the off-resonance is centrally located in the imaging field of view and has a limited spatial constraint. Therefore, a priori knowledge of the susceptibility position and spatial extent is required.
In addition to lengthy scan times, MAVRIC also utilizes a frequency-encoding (readout) gradients, which fundamentally limits its ability to eliminate in-plane signal loss and pile-up. While VAT and Jacobian methods help reduce in-plane signal loss and pile-up errors near metal, signal loss is unavoidable when the local B0 gradient within a voxel exceeds the readout gradient, such as in tissue directly adjacent to a metal object. MAVRIC also uses a high readout bandwidth in combination with narrow spectral excitation bins to mitigate, but not eliminate, errors due to frequency encoding; however, this results in a reduction of signal-to-noise ratio (“SNR”). In addition, using a narrow RF excitation bandwidth requires a greater number of acquisitions to excite the same range of off-resonance frequencies.
Single point imaging (“SPI”) techniques encode k-space one point at a time by eliminating frequency-encoding gradients and have been previously proposed in an effort to produce distortion-free images in the presence of off-resonance. This effort, however, has not gained traction because of prohibitively long scan times associated with SPI methods. A recent SPI method that is capable of spectrally-resolved, purely phase-encoded three-dimensional acquisitions was recently proposed, as described in co-pending U.S. patent application Ser. No. 13/451,773, filed on Apr. 20, 2012, entitled “System and Method for Spectrally-Resolved Three-Dimensional Magnetic Resonance Imaging Without Frequency-Encoding Gradients,” and which is herein incorporated by reference in its entirety.
Thus, there remains a need for a system and method for magnetic resonance imaging that is capable of accelerating data acquisitions in the presence of severe off-resonances, such as those caused by magnetic field inhomogeneities induced by a metallic object.